From Flashlights to Lasers: How Light Carries Our Data
By Amrita Gill and Aliesha Aden
Imagine Jack and Jill on a hike, stranded on opposite sides of a broken bridge suspended over a deep cavern. With no mobile service and night falling fast, their only way to communicate is with flashlights. Jack switches his light on and off to send messages across the gap, while Jill decodes the flashes on the other side.
At first glance, it’s a simple exchange. But this basic idea lies at the heart of one of the most advanced communication systems we use today called optical or laser communications. Just as Jack’s flashes represent a kind of visual Morse code, scientists use light to carry digital information across space, between satellites, or through the atmosphere at incredible speeds.
Turning Light into Data: The Basics of Optical Modulation
In the world of laser communications, data isn’t sent as electrical signals through wires, it’s transmitted as light. To make this possible, the light source (often a laser) must be modulated, meaning its properties are varied in a controlled way to represent the digital 1s and 0s that make up all electronic information.
When Jack switches his flashlight on, that bright pulse of light represents a binary 1. When it’s off, that absence of light represents a 0. This simplest form of modulation is called on–off keying (OOK), and it’s a type of intensity modulation (IM) where data is encoded by changing the intensity, or brightness, of the beam.
In this setup, Jack acts as the transmitter, sending out the light pulses, while Jill is the receiver, decoding what she sees. To interpret the signal, Jill sets a threshold level to distinguish between “light” and “no light.” Anything above that threshold is read as a 1; anything below it, a 0. This process of reading variations in light intensity is known as direct detection (DD).
When the Air Gets in the Way
Light travels fast and straight, but Earth’s atmosphere is far from a perfect medium. It’s filled with turbulence, moisture, dust, and temperature variations, all of which distort the path of a laser beam as it moves through the air.
As the beam travels farther, it begins to scatter and fade. Returning to our analogy, even if Jack’s flashlight starts out bright, by the time it reaches Jill, the light might have dimmed enough to fall below her detection threshold. She might see a flicker and mistake it for “no light,” introducing an error in the message.
These distortions, known as fading, are a fundamental limitation of intensity-modulated, direct-detection systems. They reduce the accuracy of the data being received and limit the range over which the system can operate reliably. The further the light travels through turbulent air, the higher the chance of errors creeping into the signal.
Adding a New Dimension: Phase Modulation
To overcome this limitation, scientists use another approach, phase modulation (PM). Instead of encoding information in how bright the light is, phase modulation encodes it in the phase of the light wave essentially, where the peaks and troughs of the wave fall in time.
Even if the brightness of the light fades due to atmospheric conditions, its phase information can still be detected, making this method much more resilient over long distances.
The simplest version is binary phase shift keying (BPSK), where binary 1s and 0s are represented by two phases of the light wave that are 180° apart. More complex versions can divide the phase into four or more distinct states, allowing a single light wave to carry multiple bits of information at once. This means more data can be transmitted without increasing the power of the laser.
The Role of the Receiver
The main difference between intensity and phase modulation lies in the kind of receiver used.
For intensity modulation, Jill only needs to measure the light’s brightness, a direct detection system. But phase modulation requires something more advanced: a coherent receiver.
A coherent receiver works by introducing a second laser at the receiving end, known as a local oscillator, which serves as a reference. When the incoming signal and the reference laser are combined, their phase difference reveals the encoded data. This process allows the system to detect even very faint signals, since it’s not relying on the beam’s intensity alone.
In short, coherent detection can dramatically improve both data capacity and transmission distance, making it ideal for deep-space communications or high-speed ground-to-satellite links.
How TeraNet is Pushing the Boundaries
Western Australia’s TeraNet optical ground station network is designed to explore and advance exactly these kinds of technologies. All three telescopes in the network located across WA’s Mid West and metropolitan regions, are equipped for direct detection, allowing researchers to work with intensity-modulated signals.
However, TeraNet-2 (TN-2) goes a step further: it also supports coherent detection, enabling experiments that compare both modulation formats in real-world atmospheric conditions. This unique capability allows the TeraNet team to test how different signal types behave under turbulence, dust, and varying weather. These insights that are essential for developing the next generation of optical communication systems.
By studying how light carries information through Earth’s atmosphere, researchers can improve the reliability of future ground-to-space laser communication links, where distance, signal loss, and atmospheric distortion all play critical roles.
Looking Ahead
From Jack and Jill’s flashlight signals to precision laser links that can one day connect Earth and orbiting satellites, the principle remains the same, using light to send information.
As systems like TeraNet continue to evolve, the lessons learned from these optical experiments will help shape the future of high-speed, high-capacity communication, not just across our planet, but between planets.